Mechanisms for high spin in black-hole neutron-star binaries and kilonova emission: inheritance and accretion

Abstract

Black-hole neutron-star binary mergers, whose existence has been confirmed by gravitational-wave detectors, can lead to an electromagnetic counterpart called a kilonova if the neutron star is disrupted prior to merger. The observability of a kilonova depends crucially on the amount of neutron star ejecta, which is sensitive to the aligned component of the black hole spin. These binaries likely originate from the evolution of isolated stellar binaries. We explore the dependence of the ejected mass on two main mechanisms that provide high black hole spin. When the black hole inherits a high spin from a Wolf-Rayet star that was born with least ∼ 10% of its breakup spin under weak stellar core-envelope coupling, which is relevant for all formation pathways, the median of the ejected mass is 10 −2 M . Though only possible for certain formation pathways, similarly large ejected mass results when the BH accretes 20% of its companion's envelope to gain a high spin, and a more massive stellar progenitor provides smaller ejected mass compared to when the black hole inherits high spin. Together, these signatures suggest that a population analysis of black hole masses and spins in black-hole neutron-star binary mergers may help distinguish between mechanisms for spin and possible formation pathways. Using a novel kilonova light curve model we show that current capabilities are unlikely to observe a counterpart, however future facilities such as the Vera Rubin Observatory will likely detect counterparts even if the aligned dimensionless spin of the disrupting black hole is as low as ∼ 0.2. Our model predicts kilonovae as bright as M i ∼ −14.5 for an aligned black hole spin of ∼ 0.9.